Why not just precede your list of options with some "shared" text such as "Select your preferred subscription method", then just call them email, rss, facebook, twitter, googleplus?
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FumbleFingersApr 2 '12 at 18:18

Note that subscribe is not the usual word to use for those platforms. You follow someone on Twitter, you add (or like, depending on what you're looking for people to do exactly) someone on Facebook, and you add (or +1) someone on Google+. Neither platform offers any kind of subscription, so using subscribe seems quite misguided to begin with.
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Janus Bahs JacquetNov 24 '14 at 9:18

1 Answer
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Following FumbleFingers rewording suggestion would be the most sensible thing to do. But let me address your «Can I use "by" instead of "using"» question: While use of by would not break any rules of grammar or meaning and would be commonly understood, I would not choose to use it in the way your examples show. I regard "subscribe using twitter" and "subscribe via twitter" as acceptable and nearly equivalent, but "subscribe by twitter" as awkward. (Via has senses as a preposition of "by way of" and "by means of").